Many communication systems of today use Frequency Hopping (FH) of a carrier wave in order to spread the frequency spectrum and make it look more like noise.
Information is transmitted by modulating a carrier wave. Modulation is defined as the process by which some characteristic of a carrier is varied in accordance with a modulating signal. In digital systems the modulating signal consists of a stream of binary data or an M-ary encoded version of it. The carrier is usually a sinusoidal signal. The carrier can be modulated by using Amplitude-shift keying (ASK), Frequency-shift keying (FSK) or Phase-shift keying (PSK) or combinations of these modulation principles as e.g. Quadrature Amplitude Modulation (QAM) or Orthogonal Frequency Division Multiplexing (OFDM). In this way a number of signals can be created at the carrier frequency. In the case of M-ary signalling a set of M signals are created where M=2m and m is the number of bits of source data that is transmitted at a time. Binary signalling is a special case where m=1 and thus M=2, i.e. one bit of source data is transmitted at a time using two signals. In the case of binary FSK two frequencies are used, one representing 0, also called the space frequency, and one representing 1, also called the mark frequency. If two bits are transmitted at the same time m=2 and M=4, i.e. 4 signals are generated. The two bits of source data transmitted at a time when m=2 is called a symbol and the symbol length here is two bits.
In frequency hopping (FH) systems the carrier frequency is changed, or hops, in accordance with a predefined frequency pattern. This pattern appears to change at random but is actually a pseudo random pattern generated by a pseudo random noise generator, called a PN-code generator. A common modulation type for FH systems is M-ary frequency-shift keying (MFSK). There are two types of Frequency Hopping:                Slow Frequency Hopping (SFH) where more than one symbol is transmitted within one frequency hop, which means that the symbol rate is an integer multiple of the hop rate, and        Fast Frequency Hopping (FFH) where one symbol includes more than one frequency hop, which means that the hop rate is an integer multiple of the MFSK symbol rate.        
In FH systems the spectrum of the transmitted signal is spread by the randomly hopping in frequency. The hopping frequency in FFH systems is higher than the symbol frequency and can also be higher than the bit frequency. The amount of spreading is proportional to the hopping frequency.
Normally the communication systems are of duplex type i.e. a two way communication with a transmitter and receiver at each end of the communication link. In order to synchronize transmitter and receiver the transmission is initially started at one single, predefined frequency. The receiver listens at this known frequency and when the predefined PN-code is detected by the receiver, synchronization is made and reception of data can start. The transmitter at the receiving end can also send an acknowledgement to the receiver at the transmitting end that synchronization has been made and that transmission of data can start. When the synchronization is established Frequency Hopping can also be started. The disadvantage with this solution is that it makes it relatively easy for a jammer to disturb the communication when the receiver is trying to establish synchronization by listening at one single frequency.
When FH communication takes place between two platforms moving in relation to each other there is a need for a frequent synchronization of transmitter and receiver. The situation becomes even more complicated when there is a one way communication and thus not possible to acknowledge to the transmitter that synchronization has been made.
There is thus a need for improved synchronization in FH communication systems not sensible to jamming and also suitable for one way communication.